pagination util

PaginationUtil.java

import java.util.Comparator;
import java.util.stream.IntStream;

public class PaginationUtil {
    private final int pageNumber;

    private final int blockSize;

    private final int pageCount;

    /**
     * constructor
     *
     * @param totalCount total count
     * @param pageNumber page number
     * @param pageSize page size
     * @param blockSize block size
     */
    public PaginationUtil(int totalCount, int pageNumber, int pageSize, int blockSize) {
        // page number can not be minus
        pageNumber = Math.max(pageNumber, 1);
        this.pageCount = (int) Math.ceil(totalCount * 1.0 / pageSize);
        this.pageNumber = Math.min(this.pageCount, pageNumber);
        this.blockSize = Math.min(this.pageCount, blockSize);
    }

    public boolean isFirst() {
        return this.pageNumber == 1;
    }

    public boolean isLast() {
        return this.pageNumber == this.pageCount;
    }

    public boolean hasPrevious() {
        return !isFirst();
    }

    public boolean hasNext() {
        return !isLast();
    }

    public int previousNumber() {
        return hasPrevious() ? this.pageNumber - 1 : this.pageNumber;
    }

    public int nextNumber() {
        return hasNext() ? this.pageNumber + 1 : this.pageNumber;
    }

    public int[] getBlock() {
        int currentIndex = isFirst() ? 0 : (isLast() ? this.blockSize - 1 : (this.blockSize - 1) / 2);

        int[] block = new int[this.blockSize];

        block[currentIndex] = this.pageNumber;

        IntStream.range(0, currentIndex)
                 .boxed()
                 .sorted(Comparator.reverseOrder())
                 .forEach(i -> block[i] = block[i + 1] - 1);

        IntStream.range(currentIndex + 1, this.blockSize).forEach(i -> block[i] = block[i - 1] + 1);

        return block;
    }

    public int getPageNumber() {
        return pageNumber;
    }

    public int getPageCount() {
        return pageCount;
    }
}

PaginationUtilTest.java

import static org.junit.jupiter.api.Assertions.assertArrayEquals;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertFalse;
import static org.junit.jupiter.api.Assertions.assertTrue;

import org.junit.jupiter.api.Test;

public class PaginationUtilTest {

    @Test
    public void testPagination() {
        PaginationUtil pager = new PaginationUtil(100, 4, 2, 5);
        assertArrayEquals(new int[]{2, 3, 4, 5, 6}, pager.getBlock());
        assertEquals(4, pager.getPageNumber());
        assertEquals(50, pager.getPageCount());
        assertEquals(3, pager.previousNumber());
        assertEquals(5, pager.nextNumber());

        pager = new PaginationUtil(8, 2, 2, 3);
        assertArrayEquals(new int[]{1, 2, 3}, pager.getBlock());

        pager = new PaginationUtil(8, 3, 2, 3);
        assertArrayEquals(new int[]{2, 3, 4}, pager.getBlock());

        pager = new PaginationUtil(8, 4, 2, 3);
        assertArrayEquals(new int[]{2, 3, 4}, pager.getBlock());

        pager = new PaginationUtil(8, 4, 200, 3);
        assertArrayEquals(new int[]{1}, pager.getBlock());
    }

    @Test
    public void testFirstBlock() {
        PaginationUtil pager = new PaginationUtil(8, 1, 2, 1);
        assertArrayEquals(new int[]{1}, pager.getBlock());
        assertTrue(pager.isFirst());
        assertFalse(pager.isLast());
        assertFalse(pager.hasPrevious());
        assertTrue(pager.hasNext());
        assertEquals(1, pager.getPageNumber());

        pager = new PaginationUtil(8, 1, 2, 2);
        assertArrayEquals(new int[]{1, 2}, pager.getBlock());

        pager = new PaginationUtil(8, 1, 2, 3);
        assertArrayEquals(new int[]{1, 2, 3}, pager.getBlock());
    }

    @Test
    public void testLastBlock() {
        PaginationUtil pager = new PaginationUtil(10, 5, 2, 1);
        assertArrayEquals(new int[]{5}, pager.getBlock());

        pager = new PaginationUtil(10, 5, 2, 2);
        assertArrayEquals(new int[]{4, 5}, pager.getBlock());

        pager = new PaginationUtil(10, 5, 2, 3);
        assertArrayEquals(new int[]{3, 4, 5}, pager.getBlock());

        pager = new PaginationUtil(10, 5, 2, 4);
        assertArrayEquals(new int[]{2, 3, 4, 5}, pager.getBlock());

        pager = new PaginationUtil(10, 5, 2, 5);
        assertArrayEquals(new int[]{1, 2, 3, 4, 5}, pager.getBlock());
        assertFalse(pager.isFirst());
        assertTrue(pager.isLast());
        assertTrue(pager.hasPrevious());
        assertFalse(pager.hasNext());
        assertEquals(5, pager.getPageNumber());
    }
}